Recent interest in portable wireless communication systems has prompted much research in the design of efficient radio frequency (RF) receivers. The need for receiver portability, however, limits the available battery power, and consequently places severe constraints on the power consumption, physical size and weight of such devices. Miniature radio receivers dissipating low power, therefore, are highly desirable. This search for efficient RF receivers has resulted in a resurgence of interest in simplified architectures.
About 98% of existing RF receivers are based on the superheterodyne architecture, which is shown in its simplest form in FIG. 1. An antenna 20 couples to an RF signal and feeds it to an RF amplifier 22. The amplified RF signal is then converted to an intermediate frequency (IF) by mixing it with a signal produced by an offset local-oscillator 24. The resulting IF signal is then substantially amplified by an IF amplifier 26 and then shifted to baseband by mixing it with a signal from a second local oscillator 28. The baseband signal is then quantized in an analog-to-digital (A/D) converter 30 and demodulated by a digital signal processing (DSP) demodulator 32.
There are several disadvantages of the superheterodyne architecture which make it impractical for low-power implementation. In order for IF amplifier 26 to produce sufficient gain in the IF signal, it must contain IF filters biased at large currents, thereby causing substantial power dissipation. Furthermore, these IF filters require numerous passive components which can not be integrated onto a single chip with the rest of the receiver, adding to receiver size and cost. Another significant drawback of the superheterodyne architecture results from the symmetry in mixing the RF signal with the signal from offset local oscillator 24. In addition to the desired RF signal, this mixing produces undesired image signals at an intermediate frequency above or below the offset local oscillator frequency. Removing the image signals, however, requires a more complicated and expensive receiver design, e.g. a very selective and expensive analog RF filter, or two or more IF stages.
The direct conversion receiver architecture, shown in FIG. 2, avoids many of the above difficulties of the superheterodyne architecture. An antenna 34 couples to an RF signal and feeds it to an RF amplifier 36, as before. The amplified RF signal is then converted directly to baseband (hence the term "direct conversion") by mixing it with a signal produced by an offset local-oscillator 38. The resulting baseband signal is then substantially amplified by a baseband amplifier 40 and then quantized in an analog-to-digital (A/D) converter 42 and demodulated by a digital signal processing (DSP) demodulator 44.
Because the down-converted signal in the direct conversion design is centered at frequency zero, there is no image signal to be rejected. Consequently, the analog filtering problem can be easily handled. In addition, the direct conversion architecture relaxes the selectivity requirements of RF filters and eliminates all IF analog components, allowing for a highly integrated, low-cost and low-power receiver. Due to these and other potential advantages, direct conversion designs have been the subject of numerous recent publications. See, for example, Abidi, A., "Direct-Conversion Radio Transceivers for Digital Communications," in IEEE Journal of Solid-State Circuits, vol. 30, no. 12, December 1995; Wilson, J., et al., "A Single-Chip VHF and UHF Receiver for Radio Paging," in IEEE Journal of Solid-State Circuits, vol. 26, no. 12, December 1991; Cavers, J., et al., "Adaptive Compensation for Imbalance and Offset Losses in Direct Conversion Transceivers," IEEE Transactions on Vehicular Technology, vol. 42, no 4, November 1993; Estabrook, P., et al., "The Design of a Mobile Radio Receiver Using a Direct Conversion Architecture," in Proc. IEEE Vehicular Technology Conference, San Francisco, May 1989, pp. 63-72.
There are two serious and well-known problems associated with direct conversion designs, namely, 1/f noise and DC-offset noise. Both of these noise sources result in severe performance degradation, and, in particular, reduce the detectability of the transmitted signal. See, for example, Abidi, A., "Direct-Conversion Radio Transceivers for Digital Communications," in IEEE Journal of Solid-State Circuits, vol. 30, no. 12, December 1995; Estabrook, P., et al., "The Design of a Mobile Radio Receiver Using a Direct Conversion Architecture," in Proc. IEEE Vehicular Technology Conference, San Francisco, May 1989, pp. 63-72.
The 1/f noise (also known as flicker noise or pink noise) is an intrinsic noise phenomenon found in semiconductor devices, with a power spectral density inversely proportional to frequency. The coupling of 1/f noise with the received signal takes place primarily after down-conversion at the baseband amplifier 40 (FIG. 2) after the down-conversion. Since the baseband signal could be in the range of hundreds of microvolts rms, the 1/f noise comprises a substantial fraction of the signal power, resulting in large signal distortion. In a superheterodyne architecture, on the other hand, the IF signal is substantially amplified by IF amplifier 26 (FIG. 1). Since the IF frequency is high enough that 1/f noise is negligible, the 1/f noise then becomes relatively insignificant when the IF signal is translated to baseband.
The DC-offset noise is an offset voltage that appears in the signal spectrum at DC when an RF signal is converted directly to baseband. This offset value typically dominates the signal by as much as 50 to 100 times, and can substantially degrade the signal to noise ratio (SNR) if it is not removed. Furthermore, this offset voltage must be removed in the analog domain prior to sampling, because it would otherwise saturate the baseband amplifiers and require an A/D converter with an impractically large dynamic range.
The DC-offset noise arises from two major sources. The first source is transistor mismatch in the signal path between the mixer and the I and Q inputs of the detector. With careful circuit design, this effect could be largely minimized. The second cause of DC-offset occurs when the signal from local oscillator 38 (FIG. 2), which is at the same frequency as the RF signal, leaks from antenna 34 and reflects off an external object and self-converts to DC. This local oscillator radiation also interferes with other nearby receivers tuned at the same frequency. Since this radiation is generally many orders of magnitude stronger than the RF signal, this self-rectification and nearby interference introduce tremendous DC-offset noise after direct-conversion. Furthermore, the amount of DC-offset generated by the local oscillator radiation is difficult to predict since its magnitude changes with receiver location and orientation. Good circuit isolation techniques could reduce this effect to a certain extent, but it cannot be eliminated entirely.
Despite the potential benefits of direct conversion receivers, the serious 1/f noise and DC-offset noise problems described above have limited its development and widespread use. There are a few published methods, however, that propose to overcome some of the above mentioned problems with direct conversion. One approach, for example, is disclosed in D. Haspeslagh et al. "BBTRX: A baseband transceiver for zero IF GSM hand portable station", Proc. of Custom IC Conf., San Diego, Calif., 1992, pp. 10.7.1-10.7.4. This paper describes an approach for removing the DC offset noise by averaging the digitized baseband signal over a window and subtracting an estimate of the DC-offset noise from the signal using a D/A converter and various extraneous analog components. This method is relatively successful in nulling out DC offset but the settling time of the offset subtraction circuit may cause loss of the first few symbols in a TDMA receiver. In addition, this method fails to adequately suppress the 1/f noise. Moreover, the complexity of this approach compromises primary potential benefits of direct conversion design, namely low power and low complexity.
Wilson, J., et al., "A Single-Chip VHF and UHF Receiver for Radio Paging," in IEEE Journal of Solid-State Circuits, vol. 26, no. 12, December 1991 describe an application of direct conversion design to paging. Direct conversion has proven practical in this case because paging uses very simple two-tone signaling, i.e. wideband frequency shift keying (FSK) modulation, with the resulting spectrum having little DC energy. Since most of the distortion described above are concentrated near DC, an analog DC-notch filter which is attainable by simple capacitive coupling allows most of the noise near DC to be removed with minimal distortion to the signal spectrum.
Although the simplicity of the pager application maintains the advantages of a direct-conversion architecture, simple two-tone signaling is not suitable for most RF communications applications since these applications require modulation schemes that are more spectrally efficient per dimension than wideband FSK. In these more efficient modulation schemes, the method of capacitively coupling the baseband signal before sampling fails because of the large signal-bearing spectrum near DC, and a notch filter at DC will remove significant portions of the signal.